1. Field of the Invention
This invention relates to wire bonders and capillary or bond head orientation with respect to each other.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, wiring between two bond pads on a chip or between a bond pad on the chip and an external bonding location, such as a lead frame, is generally performed by making a ball bond on one bonding pad at one end of a wire fed from a spool and then making a stitch bond at the other end of the wire. The wire is also severed from the spool from which it has been fed at the stitch bond location. Such bonding and wire severing is generally performed with the aid of a capillary through which the wire to be bonded is passed from the wire spool. The procedure is to form a ball from a portion of the wire which extends out of the capillary, bond the ball to a bond pad and move the capillary to the bonding location with the wire being fed out through the capillary during travel of the capillary to the second bonding location. The wire is then stitch bonded at the second location, usually a lead finger, and prepared for severing from the spool, using the nose of the capillary to perform the stitch bonding task.
In order for the capillary to perform the function of stitch bonding, the capillary must apply a force against the wire which rests on the bonding location with the nose or terminal portion thereof. In order to perform this task without fracture of the capillary, as a first condition, the capillary wall must have sufficient thickness to withstand the forces applied thereto at the location of force application during the stitch bonding procedure.
This has been accomplished in the prior art by providing a wire bonding capillary with central bore or aperture, a circular cross section and wall thickness from exterior surface to central bore sufficient to accomplish the above described purpose and withstand the expected maximum forces thereon.
With the continual decrease in the spacing dimensions between wire bonding locations, particularly in the semiconductor art, the problem of bonding wire to a wire bonding surface with a capillary and then moving the capillary to a new location without interfering with adjacent wire bonding locations and wires bonded at adjacent wire bonding locations has increased. As the dimensions decrease, the possibility that the capillary will interfere with or strike an adjacent bond pad or wire extending from an adjacent bond pad in its travel from one bonding location to a second bonding location increases.
A prior art technique that has been developed to accommodate and/or minimize this problem has utilized a wire bonding capillary of circular cross section with a portion of the capillary wall on a pair of opposing sides of the capillary removed to provide an indentation or flattening thereat. Capillaries of this type are provided by Texas Instruments Incorporated under the trademark BowTI.TM.. This is accomplished by using a capillary having a nose or stitch face somewhat in the shape of a figure "8" with a hollow center to carry the wire as in the prior art and with a thin walled waist region. The thin wall portion of the figure "8" can be escalloped or a pair of flat parallel surfaces. A capillary with this shape is still capable of performing the functions of forming and bonding the ball from the wire passing therethrough at a bond pad and then stitch bonding the wire at a bond location, using the thicker-walled portion of the figure "8"-shaped capillary. The top and bottom portions of the figure "8" must be used to make the stitch bond because they are thicker and better capable of withstanding the forces applied to the capillary during stitch bonding. With a capillary having the above described shape, when bonds are to be made concurrently at adjacent bond location, the adjacent bonds are made with the capillaries rotated by 90.degree. relative to each other so that the circular or flattened waist portion of one capillary fits into but is separated from the waist portion of the adjacent capillary. As is apparent, the bond spacing can be materially decreased with the above described technique. After ball bonding one end of the wire extending from the capillary, the capillary with wire therein is moved in either an x-direction or a y-direction or both to the next bonding location during which time the wire is passed through the capillary. The other end of the wire just bonded is then stitch bonded at the next bonding location using the thicker portions of the capillary to make the stitch bond with the capillary moving in only one of the x- or y direction and in the direction of its major axis when making the stitch bond so that the thick portion of the nose of the capillary forms the stitch bond.
A problem that arises in the use of the above described figure "8" shaped capillary is that the capillary is capable of performing a bond along only one axis due to its rigidity along the major axis and fragility along the minor axis or waist of the figure "8". For this reason, for an area of coverage, the wire bonders require two bond heads or capillaries, one for operation in the x-direction and one for operation in the y-direction. However, when the device being fabricated requires that more bonds be made in one of the x or y directions relative to the other, such as, for example, when the semiconductor die is rectangular and not square, the time required for completion of the bonding operation is extended. This is a result of the fact that one of the bonding heads must make more bonds than the other, leaving one bonding head idle while the other continues to be operated.